Around here, Lowes or Home Depot has a ceramic repair kit available, usually in the paint department. My friend that does ceramic repair professionally says that a homeowner repair will usually only last a year or two. Professionally reglazing the entire tub is still much cheaper than remove & replace.
hth, Doug

I fix large chips in the enameled steel tubs in my building in a way that I think is both water proof and reasonably permanent.

1. Remove any rust from the area using a Dremel and rotary brush with stainless steel wire bristles. You can also use a 10 to 20 percent hydrochloric acid based toilet bowl cleaner to dissolve the rust. Apply it with Q-tips.

2. Wipe the area down with a wet and then damp sponge to remove any rust dust, and stop the acid from working after the rust is gone.

3. immediately flush the area with either isopropyl alcohol, rubbing alcohol or acetone. Then wipe up the alcohol or acetone with a paper towel. (That's cuz liquid water is highly soluble in alcohols and acetone. By flushing the area with alcohol or acetone and wiping up that alcohol or acetone, you're causing any water remaining on the surface to dissolve in the alcohol or acetone and then wicked up by the paper towel. When the alcohol or acetone evaporates, that area SHOULD BE perfectly dry.

4. Mix up some 2 part epoxy and apply it to the affected area, spreading it as necessary to cover the affected area.

5. Now cover the epoxy with a piece of Saran Wrap.

6. Dip your fingers into some dishwashing detergent and work the epoxy into the areas you want it to cover. The dish washing detergent will both act like a lubricant and a shock absorber, allowing you to smooth out the epoxy under the cling wrap so that it's very smooth and covering the area you want covered.

If your finger feels a "bump" where the epoxy is, then you have too much epoxy and you rectify that by simply pulling off that piece of Saran Wrap and putting down a new piece of Saran Wrap. Some of the epoxy will come up with the Saran Wrap, leaving less of it on the tub.

7. Once you get the epoxy both smooth and flush or shy-of-flush with the tub, leave it overnight like that. In the morning the Saran Wrap (or any cling wrap, really) will pull of the repair cleanly. If it doesn't, the wrap is so soft compared to the epoxy that you can scrape the wrap off the epoxy with your fingernail.

8. Now, paint over the epoxy with a tub chip repair paint like Porc-a-Fix, which makes small bottles of alkyd paint in all the colours that Crane and American Standard plumbing fixtures ever came in. Porc-a-Fix is made by:
KIT Industries, 1262 Glen Avenue, Moorestown, New Jersey, 08057
ph. 1-800-526-3186

You can phone them and find out who retails their products in your area.

The Porc-a-Fix comes in a bottle with a small piece of sandpaper. Throw the sandpaper in the garbage where it belongs and simply thin the Porc-a-Fix paint with 10 to 20 percent mineral spirits and put on thinner coats with an artist's paint brush. Thinning the paint will both allow it to self level faster and provide a longer drying time so that it has more time to self level. Thus, thinning the paint will help immensely to eliminate brush strokes.

If the repair spots are smaller than an inch in diameter, you can eliminate brushing altogether. Just spread the thick Porc-a-Fix paint over the chip, allow the paint to dry for 3 or 4 hours until it's stiff, and then shave the paint flush with the bathtub's surface with a razor scraper (the kind sold in paint stores for scraping paint off glass).

Your local hardware store will carry "Marine" epoxies. These are epoxies that may be applied underwater, and are intended to be used where they would normally be submerged. I have used both regular and marine epoxies for fixing badly chipped enamel steel tubs, and it doesn't seem to make any difference whether you use the marine or regular epoxy to repair the tub chip.

You see, the rust seems to be occuring both on the exposed enamel as well as under the chrome flange of the drain. The problem would be in ensuring water didn't continue to get at the bare steel under that chrome drain flange.

I think all you can do in this situation would be to try and repair it as described in my post, and hope that it extends the life of your tub as much as possible. You'd also want to clean the seam around the chrome flange well, and try sealing it with silicone caulk (or epoxy).

Never use hydrochloric acid on chrome; it'll attack and etch the chrome plating. Phosphoric acid is commonly used in toilet bowl cleaners and bathroom cleaners because it cuts through soap scum aggressively but doesn't harm chrome plating.

If you have the opportunity to replace this tub, I'd probably replace it since it looks to me like the rust right around the drain would be very difficult to repair, and you'd have little confidence that the rusting wasn't continuing under the chrome drain flange anyhow.

I wish I had seen the post before I redid my bathroom in 2010.. Lol I put a new American Standard/bootzcast steel enameled tub in. I'm noticing now that it appears to have some very slight scratches with very slight rust peering through, Think I am going to repair it because I had done a real nice tile job over the tub! >.<